We have been running ESXi, since it has been released as free.
But we have just started to think about licensing, with some research, I found this.
VM/ETC ( http://vmetc.com ) writes about ESXi licensing.
Yes, VMware has made a version of it’s Enterprise class, bare metal installable hypervisor available for download at no charge. There is no trial period or demo license required. There are no guest limitations on the host. So, there has to be a “gotcha”, right? Not really, but you do need to understand the differences between the free ESXi and VI ESXi (licensed) versions.
Here is the full article. ( http://vmetc.com/2008/08/10/whats-the-difference-between-free-esxi-and-licensed-esxi/ )
Here is a PDF with VI 3 Pricing from Vmware.com ( http://www.vmware.com/files/pdf/vi_pricing3.pdf )